Psarolepis
| Psarolepis | ||||||||||||
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A somewhat free reconstruction of Psarolepis . Length of the individual approx. 25 cm |
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| Temporal occurrence | ||||||||||||
| Pridolium (Upper Silurian) to Lochkovian (Lower Lower Devonian) | ||||||||||||
| 419 to 411 million years | ||||||||||||
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| Scientific name | ||||||||||||
| Psarolepis | ||||||||||||
| Yu , 1998 | ||||||||||||
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Psarolepis is a very original genus of the meat finisher (Sarcopterygii) from the youngest Silurian and the oldest Devonian . The type and only known species is Psarolepis romeri . Their fossil remains were first discovered in 1984 in China ( Yunnan Province) (X. Yu 1998) and are among the oldest nearly complete finds by sarcopterygians.
etymology
The generic name Psarolepis is derived from the ancient Greek words ψαρός ( psarós ), which means something like “ starry ” in the sense of “piebald”, “speckled”, and λεπίς ( lepís ) for “fish scale”. The epithet of the type species is intended to remind of the anatomist and paleontologist Alfred Sherwood Romer (1894–1973).
Importance of the find
Psarolepis was the first find that was well enough preserved to finally give an idea of the very primitive Sarcopterygians, when they were still quite similar to the contemporary representatives of the Actinopterygians , their sister group. This sister group relationship had been well established for a long time due to the findings of isolated teeth, spines and scales ( ichthyolites ), but it was now possible to examine such around 415 million year old (i.e. upper Silurian- lower Devonian ) fish as a whole. In the meantime, fossils from Vietnam have also been identified as Psarolepis and only the tail region is still unknown. The hope initially expressed that Psarolepis would shed a lot of light on the origins of the basal fish has not been fulfilled. The sarcopterygian Guiyu oneiros , who was around five to ten million years older and who also clearly shows some characteristics of actinopterygians , has been known from the same site in southern China since 2009 .
features
The most prominent feature of the psarolepis is a tight group of long parasymphysial teeth at the tip of the lower jaw. Opposite the whorl , the unpaired rostral in the upper jaw is somewhat indented. Such tooth whorls were already known, in a less noticeable form, from Onychodontiformes , to which Psarolepis has therefore long been attributed (Long 2001). Similar lower jaw dentition can recently be found in the Umberfish genus Macrodon , it can be interpreted as an adaptation to the crustacean diet (e.g. crabs - at that time perhaps merostomids ). Tooth whorls at the tip of the mouth have long been known, especially from fossil sharks . The remaining teeth at the edge of the mouth are smaller (of different sizes). The teeth in the oral cavity itself are at best insignificant (squeezing). The teeth themselves are of the polyplocodontic type; H. the dentine is wrinkled on the inside in a characteristic way as in other early sarcopterygians and (similarly) in the early tetrapods, the " labyrinthodontia ". The gill basket was probably cartilaginous, which is why it has not survived in similar finds. On the basis of the narrow gill covers, however, it can be deduced that only little space was available for the gills. The oxygen consumption, i.e. H. the activity of these fish was apparently generally lower than that of today.
Also noticeable are the rigid bone spines in front of the pectoral fins and the dorsal fin, the exact location of which cannot be clearly determined. This feature is also unusual for both Sarco and Actinopterygians, and is more likely to be found in placodermi , cartilaginous fish and acanthodii . However, this cannot be used as an indication of a closer relationship with these groups, as it can certainly be a matter of apomorphies , similar to recent armor catfish (Loricariidae). The bones of the snout region and the lower jaw are particularly reminiscent of actinopterygic conditions (five Infradentalia; Ahlberg 2001).
The skull, largely ossified in front and above, is, as in all original sarcopterygians, clearly divided into two roughly equal parts: the ethmosphenoid and otoccipital blocks. This improves the probability of success in catching prey, but is disadvantageous for free swimming, which is why this condition does not occur in the actinopterygians, although it may have been present in the common ancestor of all Osteichthyes . The intracranial joint is level with the exit of the trigeminal nerve from the brain. So the Onychodontiformes were almost certainly benthopelagic (and probably had a swim bladder for that purpose ). The small eyes are to the side of the noses, just behind the premaxillaria - this position is retained for a long time in Paleozoic fish. The "rear" narins (nostrils) lay in front of the eyes, between the rostral and premaxillary , both of which were strongly bent downwards in front. Inner nostrils in the oral cavity ( choans ) were missing. Ventrolaterad of the nasal sacs was the paired rostral organ for the electrical location of prey. The “premaxillary” seems to have merged with the “ palatinum ”. The short, wide parasphenoid has a central foramen ("hole") for the drainage of the pituitary secretion and two lateral foramina for the aortic roots . The labyrinth region was not ossified anteriorly. The ethmosphenoid shield (which cannot be further differentiated) has a large parietal foramen dorsally . Psarolepis and the somewhat younger and more clearly "sarcopterygic" Achoania had , just like more original gnathostomes and early actinopterygians, eye stalks (stalked cartilage cups in the orbit , against fluttering of the eyeball; more recently in sharks).
As usual, the mouth was deeply split, the hyomandibular (not preserved!) Oriented backwards and downwards and, like the front part of the jockstrap, quite weak. Since the hyomandibel steered with two heads on the otoccipital block, the front skull was lifted “automatically” when the jaw was opened - no musculature attaches to it dorsally; the basic granial muscles, however, were active when the jaws were closed. The lower jaw has been moved forward (it is steered indirectly through the [fossilized] palatoquadratum on the anterior skull: you can still see the joint sockets) similar to Malacosteus (the "catching mask beater" - of course not so excessively, because of the gular plates). However, dismantling the skull is not a particularly good solution to the old problem of bringing the mouth opening and swimming direction in the open water to congruence - it does not, however, require complex nervous feedback in the area of the labyrinth.
Since the skull was crushed during the diagenesis by the compaction of the surrounding sediment, the cranial bones are only partially clear in their positional relationships. Of the more recent fish forms, the skull of Polypterus (Flösselhecht, the "most primitive" living actinopterygian) resembles it most . The "Squamosum", a cover bone of the Sarcopterygians, which is partially homologous to the preoperculare of the Actinopterygians , shows sections of the sideline canal in the manner characteristic of Sarcopterygians. The cover bones have a thick layer of enamel. They are perforated by numerous pores, which are probably openings for nerve tracts of the skin's sensory organs. This gives the Psarolepis skull a "pockmarked" appearance. The shoulder girdle shows a certain resemblance to that of the Placodermi, but is only partially preserved. The trunk of Psarolepis was covered by elongated cosmin scales (smaller than in the picture above!), The caudal fin most likely gephyrocerk.
Systematics
Psarolepis is a basic representative of the Sarcopterygians. In its first description it was speculated that Psarolepis might be older than the separation of the two main lines of bony fish, but this is not even the case for Guiyu . The systematic position within the basal sarcopterygians according to current knowledge shows the following cladogram :
| Sarcopterygii |
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- *According to recent findings is Meemannia the geologically oldest ray-finned and Lingualepis a core group representatives of bonefish .
literature
- Per Erik Ahlberg: Something fishy in the family tree . In: Nature 397 (6720), 1999, pp. 564-565, doi: 10.1038 / 17484 ; Reprinted pp. 64–66 in: Henry Gee (Ed.): Rise of the Dragon: Readings from Nature on the Chinese Fossil Record. The University of Chicago Press, 2001, ISBN 0-226-28491-3 ( GoogleBooks )
- John A. Long: On the relationships of Psarolepis and the onychodontiform fishes . In: Journal of Vertebrate Paleontology 21, 2001, pp. 815-820, doi : 10.1671 / 0272-4634 (2001) 021 [0815: OTROPA] 2.0.CO; 2
- Qu Qingming, Zhu Min, Wang Wei: Scales and Dermal Skeletal Histology of an Early Bony Fish Psarolepis romeri and Their Bearing on the Evolution of Rhombic Scales and Hard Tissues. In: PLoS ONE 8 (4), 2013, item no. e61485, doi : 10.1371 / journal.pone.0061485
- Yu Xiaobo: A new porolepiform-like fish, Psarolepis romeri , gen. Et sp. nov. (Sarcopterygii, Osteichthyes) from the Lower Devonian of Yunnan, China . In: Journal of Vertebrate Paleontology 18, 1998, pp. 261-274. doi: 10.1080 / 02724634.1998.10011055
References and comments
- ↑ Qu et al. (2013), p. 1
- ↑ which is why it was previously thought that they were about the "resurgence of ancient hereditary goods"; but it is much easier to speak of convergences here .
- ↑ what it exceptionally even at Elops still exist
- ↑ Min Zhu, Xiaobo Yu, Per E. Ahlberg: A primitive sarcopterygian fish with an eyestalk. In: Nature. 410, 2000, pp. 81-84, doi : 10.1038 / 35065078 .
- ↑ the two heads, however, still very close together, as in Acanthodii. See also "Palaeos"
- ↑ Yu Xiaobo, Zhu Min & Zhao Wenjin: The Origin and Diversification of Osteichthyans and Sarcopterygians: Rare Chinese Fossil Findings Advance Research on Key Issues of Evolution. In: Bulletin of the Chinese Academy of Sciences 24 (2), 2010, pp. 71–75 ( PDF )
- ↑ Jing Lu, Sam Giles, Matt Friedman, Jan L. den Blaauwen, Min Zhu: The Oldest Actinopterygian Highlights the Cryptic Early History of the Hyperdiverse Ray-Finned Fishes. In: Current Biology 26 (12), 2016, pp. 1602-1608, doi: 10.1016 / j.cub.2016.04.045